A painful lump in the groin is most often a swollen lymph node reacting to an infection nearby, and a few sexually transmitted infections, chiefly genital herpes and syphilis, can trigger that swelling along with a sore. But plenty of non-STI causes do it too, like an inguinal hernia or a skin or leg infection, so a test tells you which it is.
Herpes simplex virus
Treponema pallidum
| Item | Value |
|---|---|
| Genital herpes | managed — Herpes simplex virus |
| Syphilis | curable — Treponema pallidum |
Your groin is packed with lymph nodes, small bean-shaped filters that swell and turn tender when they're fighting an infection in the genitals, legs, or lower body. So a painful lump there usually isn't the disease itself; it's your immune system at work. The question is what set it off. Below are the STIs worth thinking about, the common non-STI explanations, and how a clinician tells them apart, which usually comes down to a test.
Which STIs cause a painful lump or swollen node in the groin
Two STIs are the usual suspects when a groin lump shows up alongside a genital sore. Each has a tell-tale pattern, but the patterns overlap enough that sight alone won't settle it.
Genital herpes
Genital herpes is caused by two viruses, herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) CDC, About Genital Herpes. When it does cause a first outbreak, it tends to be the loud one: small blisters that break open into painful sores, taking a week or more to heal, often with flu-like symptoms like fever, body aches, and swollen glands. Those swollen glands are the groin nodes you can feel, tender because the lymph tissue is inflamed. Sores can show up on or around the genitals, rectum, or mouth.
Most people with herpes have no symptoms or very mild ones, and most don't know they carry it; the majority of HSV-2 infections are never diagnosed. So a painful groin node with a cluster of small, painful sores points toward herpes, but a quiet infection can give you almost nothing to go on. Repeat outbreaks are shorter and milder than the first, and many people get a warning prodrome of tingling, itching, or burning in the area a day or so before sores appear.
Syphilis
Syphilis is caused by the bacterium Treponema pallidum and is curable with the right antibiotics CDC, About Syphilis. Its first stage produces one or more sores called chancres at the spot the bacteria entered: the penis, vagina, anus, rectum, lips, or mouth. A syphilis chancre is usually painless, firm, and round. It typically appears about three weeks after exposure, though the incubation can run anywhere from about ten to ninety days. It lasts a few weeks and then heals on its own whether or not you treat it, which is why people miss it.
So why is syphilis on a "painful lump" page at all? The sore itself is painless, but the nearby lymph nodes can swell, and in the secondary stage, swollen lymph nodes are a recognized symptom alongside a rough red or reddish-brown rash (classically on the palms and soles), fever, sore throat, patchy hair loss, headache, muscle aches, and fatigue. If you've got a firm painless sore plus a tender groin node, syphilis belongs on the list. Read more in what is syphilis? causes, stages & risks.
When it's NOT an STI
Plenty of painful groin lumps have nothing to do with sex. The two big non-STI causes to keep in mind are an inguinal hernia, a bulge that happens when tissue pushes through a weak spot in the lower abdominal wall and often becomes more obvious when you stand, cough, or strain, and a skin or leg infection, where a cut, ingrown hair, boil, or athlete's-foot-type infection on the foot or leg drains into the groin nodes and makes them swell and ache.
Other everyday culprits include irritated hair follicles and cysts. Tenderness and pain are useful clues, since they suggest active inflammation, but they don't pin down the cause. A painful lump can be any of these, and that's the whole problem.
How to tell them apart
You usually can't tell by looking. The signs overlap too much, and several of these conditions are frequently silent, so a test settles which one (if any) it is. A clinician weighs the discriminating features:
- Is there a sore, and does it hurt? Painful blistering sores lean toward herpes; a single firm, painless sore leans toward syphilis.
- Does the lump reduce or bulge with straining? That points away from an STI and toward a hernia.
- Is there a recent cut, boil, or foot infection on the same side? That suggests the node is draining a skin or leg infection.
- Are there whole-body symptoms like fever, rash on the palms and soles, or sore throat? Those raise the question of secondary syphilis.
Overlapping symptoms are why you usually can't self-diagnose this. A test turns a guess into an answer.
Painful groin lump: STI causes at a glance
| Cause | The sore | The groin node | Other clues | How it's confirmed |
|---|---|---|---|---|
| Genital herpes | Cluster of small blisters breaking into painful sores | Tender, swollen (often with first outbreak) | Flu-like symptoms; prodrome before repeat outbreaks; often no symptoms at all | Swab of the lesion (NAAT or culture) |
| Syphilis | One or more firm, round, usually painless sores (chancre) | Can swell near the chancre | Later: rash on palms/soles, fever, sore throat, hair loss | Two blood tests (nontreponemal + treponemal) |
| Inguinal hernia | No sore | Bulge, not a true node; may reduce or worsen with straining | More obvious standing, coughing, lifting | Physical exam, sometimes imaging |
| Skin/leg infection | Cut, boil, or ingrown hair nearby | Tender node draining the infected area | Redness, warmth, recent injury on same side | Exam; sometimes a wound culture |
How it's tested
If a sore is present, herpes is confirmed by a type-specific swab of the lesion using NAAT or culture, and swab-based tests work best on an actual lesion CDC, Herpes Testing. Syphilis is diagnosed with two blood tests together: a nontreponemal test (RPR or VDRL) plus a treponemal test (TP-PA, FTA-ABS, EIA, or CIA) CDC, Syphilis Lab Recommendations 2024. In practice, testing is a urine sample, a self-collected swab, or a quick exam depending on what's suspected, and it's free or low-cost at health departments, Planned Parenthood, and Title X clinics, with results usually back in a few days. When you're ready, get tested, and if your exposure was recent, check when to test after exposure so you don't test too early to catch it.
What to do next
Get the lump looked at, especially if there's a sore you can swab; that's the fastest route to an answer. Both herpes and syphilis are manageable. Syphilis is curable with the right antibiotics, and herpes can be controlled to reduce outbreaks and lower transmission CDC, Genital Herpes Treatment. Don't try to self-treat a sore or wait it out. A syphilis chancre heals on its own while the infection keeps going. For what treatment actually looks like, see CDC, Syphilis Treatment and explore alternative herpes treatments once you have a diagnosis.
Red flags — when to get seen urgently
- The lump is rapidly enlarging, hot, or the overlying skin is bright red and spreading.
- You have a high fever, chills, or feel generally very unwell.
- The bulge can't be pushed back in and is painful. A hernia that's stuck can cut off its own blood supply and is a surgical emergency.
- The lump is rock-hard, fixed in place, or growing slowly over weeks without a clear infection; get it evaluated to rule out other causes.
- There's severe pain, or the sores are spreading and not healing after a week or more.